The invention relates to a photometric measuring system and technique and, more particularly, to a system and technique for measuring amounts of chemical substances in biological fluids by using fixed time rate reactions.
Photometric analysis is widely used to determine the concentration of a chemical substance in fluids, such as biological fluids. Generally, the instruments operate on the principle that the absorption of light by the solution is directly proportional to the concentration of the solute in the solution, and that the absorption of light is independent of the intensity of the light passed through the fluid containing the solute. When biochemical substances possess characteristics which follow these two principles, photometric analysis of fluid samples having selected known concentrations of chemical substances can be employed to calibrate instruments for subsequent measurements of selected unknown sample concentrations, or to develop calibration curves for converting photometric readings directly into substance concentrations. The instrumentation that implements these principles has been severely limited because of the requirement for accurate operator calibration and accounting for variations in electrical components, the transmissibility of the reagents and other factors which affect the accuracy of the measurements.
When photometric measurements are used to produce standard curves for converting photometric readings directly into substance concentrations, readings are first taken at a variety of known concentrations. The readings are corrected by known techniques to compensate for changes caused by the disparity in the transmissibility of chemical reagents and are then plotted on graph paper at points representing the particular concentrations. Lines drawn through the majority of the points are used to determine the concentration when photometric readings are taken of unknown samples. Such curves, however, are only accurate for a particular set of reagents, or cuvettes (vials receiving the fluids) and for the specific instruments standardized at that particular time.
Thus, when new reagents and new cuvettes are used or the instrument is repaired, restandardization is necessary. Even when the same instruments are used, restandardization may be necessary due to changes in electronic components during the passing of time.
When direct read-out photometric systems are used, substantial effort is required to calibrate and adjust the instrument and to enter appropriate sealing factors required to obtain readings of certain quantities. Over time, the same readjustments and recalibrations to compensate for changes in electronic components, etc. are required to assure instrument accuracy. As a result, measurements of unknown samples are time consuming and subject to inaccuracies caused by operator error.
Other known methods of determining the chemical concentration of a substance in a biological fluid include electrolysis, resistance measurement, capacitance detection, and others. However, these methods often require calibration times that in some instances exceed the actual test time, are not as accurate, and are not operated as easily as photometric analysis.